Optical tracers are well known in the art as means for tracing patterns for controlling machines tools or similar equipment in accordance with the pattern. One class of such tracers includes those which are known as circular scanning tracers. A typical example of such a scanning tracer is shown in Canadian Pat. No. 917,773 issued Dec. 26, 1972, assigned to the assignee of the present application. The center point of the circular scan of such a machine may be referred to as its steering axis, since the line through this point vertical with reference to the plane of the pattern being traced, is the point of steering rotation of the system. Both the scanner and the cutting tool are symmetrical about such an axis, although the cutting tool will be displaced from the pattern either to an equivalent location to one side or an equivalent location vertically above or below the plane of the pattern. In any event, both the scanner and the machine tool have a reference axis with respect to which their subsequent direction of motion may be described in direction and velocity.
It is also necessary that the scanner observe the direction of the pattern in advance of the steering axis in order to provide advanced steering information so that the direction of the pattern with reference to the current position of the scanner may be used to determine the direction or progress of the machine. Recognizing that the scanner together with the drive mechanism forms part of a servo loop, it will be understood that the point of observation of the pattern by the scan with reference to the steering axis is critical to the stability and effectiveness of the servo loop. The distance between the point of observation and the steering axis is generally referred to as the lead of the machine.
Similarly, there is a further offset which is necessary in most equipment and that is the kerf offset of the tool. Most machine tools including gas torches, when cutting, remove a strip of material of a given width referred to as kerf. In order that a piece be cut to a desired size, the machine tool must not move along a pattern of the desired size, otherwise the portion of the material removed would be removed from the material being cut, reducing it in size from the size of the pattern. This offset of both the steering axis of the tracer and the steering axis of the tool is referred to as kerf adjustment. Normally speaking, this adjustment must be variable to allow for tool diameter or width of cut and also must be made positive or negative depending on whether the machine is cutting inside or outside the pattern and whether the tracing is clockwise or counterclockwise. All the foregoing problems have been recognized in the past and various alternatives have been provided to correct for lead and kerf adjustment.
A typical example of lead adjustment, as disclosed in Canadian Pat. No. 917,773 above, provided for varying diameters of circular scan by substituting differently inclined angular mirrors in a circular scanning system where the scan was produced mechanically by rotating the mirror about the scanning axis by means of a motor. Obviously, this solution requires disassembly of the scanning apparatus to change lead. Lead on the other hand really should be changed in accordance with the tracing velocity of the machine, becuase as will be understood, the important factor in a servo mechanism of this type is a time factor between the steering axis and the tracing point and if the diameter of the circle remains constant, the time factor varies depending on velocity.